I had the same problem after acquiring my '93 GLT. I must have been under the dash ten times. Then it dawned on me. I was getting all kinds of fault codes. The previous owner mentioned a new evaporator was installed. I noticed the connectors for the three damper motors were the same. Even the wires were just about the same length.

It occurred to me the connectors were mixed up during the evaporator procedure.

A little trial and error with the connectors and suddenly a miracle! All the error codes went away and the air flow problems were cured!

The mechanic who replaced the evaporator put all three connectors on the wrong motors.

I hope this info helps.

- thumbs